Piezoelectric oscillators, typified by crystal oscillators, are used heavily as frequency signal sources for communication equipment or the like because of their high frequency stability. With the continued miniaturization of communication equipment, there is a growing demand for higher frequency stability which depends on the suppression of secular variations and undesired resonances. The piezoelectric vibrator through utilization of a piezoelectric phenomenon accompanying mechanical vibrations; it is customary in the art to keep on its oscillation by supplying a current of a predetermined value or more to the piezoelectric vibrator to generate therein mechanical vibrations at levels above a certain value. It is known that the mechanical vibrations of the vibrator induce therein mechanical stress and that the long-term creation of a great mechanical distortion causes secular changes of the resonance frequency, degradation of electric resistances, deterioration of resonance characteristics under the influence of mechanical fatigue and so forth. In particular, as an oscillator required to maintain an extremely high degree of frequency stability, there is now used an oven type oscillator (OCXO) housed in a constant temperature oven so as to minimize frequency changes caused by temperature variations; such an oscillator adopts means for improving the secular variation characteristic and the undesired resonance suppression characteristic (an undesired mode oscillation suppression characteristic) by minimizing the current flow through the vibrator to restrain the induction of stress by driving.
FIG. 23 shows an example of an oscillation circuit heretofore used in OCXO.
A crystal oscillator 100 depicted in FIG. 23 is adapted to: apply the output from an oscillation circuit 101 surrounded by the broken line to an AGC circuit 104 via a capacitance 102 and a buffer circuit 103; rectify one portion of the oscillation output in the AGC circuit to generate a DC voltage; and provide the DC voltage as a base bias voltage of an oscillation transistor of the oscillation circuit 101, thereby holding the base terminal current of the oscillation transistor at a certain level. Incidentally, such a circuit is called an AGC circuit in the conventional OCXO, but in the present invention attention is rather paid to the function of a circuit which cuts down the current flow through the crystal vibrator, and hence the circuit will hereinafter be referred to as a current suppressing circuit.
In this example, the oscillation circuit 101 is a common Colpitts type oscillation circuit, in which a crystal vibrator 106 grounded at one end via a capacitance 105 is connected at the other end to the base of a transistor 107, a series circuit of capacitances 108 and 109 is connected between the base of a transistor 107 and the ground, and the connection point of the series circuit is connected to the emitter of the transistor 107 and grounded via a resistance 111.
A bias circuit for the base of the transistor 107 is formed by a resistance 110 connected between the base and the ground, and a resistance 112 for the voltage supply therethrough from the current suppressing circuit (the AGC circuit) 104 described later on. The output from the oscillator is derived via a collector load resistance 113 connected between the collector of the transistor and an AC-wise grounded power supply line Vcc and is provided via the capacitance 102 and the buffer circuit 103 as mentioned above, and one portion of the oscillator output provided to an output terminal OUT via a resistance 123 and a capacitance 124.
On the other hand, the current suppressing circuit 104 branches one portion of the output signal from the buffer circuit 103 via a series circuit of a resistance 114 and a capacitance 115, and a branched current is provided to the cathode end of a diode 116 and the anode end of a diode 117. The other ends of these diodes are individually grounded via capacitances 118 and 119, respectively, and the other ends of the diodes are interconnected via a resistance 120, and the anode end of the diode 116 is connected to the base of the transistor 107 via the resistance 112 of the oscillation circuit 101. Further, a series circuit of resistances 121 and 122 is connected between the power supply Vcc and the ground, the connection midpoint of the series circuit being connected to the cathode of the diode 117.
A description will be given below of the operation of the crystal oscillator 100 of the above configuration. Since the oscillation circuit 101 is a common Colpitts type one, the following description will not be made of its operation but given mainly of the operation of the current suppressing circuit 104.
In the first place, upon application of the oscillation output from the oscillation circuit 101 via the buffer 103 to the current suppressing circuit 104, the positive and negative half cycles of the oscillation output flow to the ground via the diode 117 and the diode 116, respectively. As the result, the voltage of the connected position between the resistance 120 and diode 116 becomes low corresponding to the oscillation output level, and the voltage of the connected position between the resistance 120 and diode 117 becomes high corresponding to the oscillation output level. That is, a voltage drop across the resistance 120 increases with an increase in the oscillation output level.
On the other hand, the base bias voltage for the oscillation transistor 107 is supplied from Vcc via a the resistance 121, the resistance 120 and the resistance 112; in this instance, since the voltage drop across the resistance 120 increases with an increase in the oscillation level, the base bias voltage drops, and consequently, the base terminal current of the transistor decreases, resulting in a decrease in the oscillation gain and a reduction in the current flow through the vibrator accordingly. That is, an increase in the current flow through the crystal vibrator 106 causes an increase in the oscillation output to be fed to the current suppressing circuit 104, producing a corresponding increase in the voltage drop across the resistance 120 and hence causing a decrease in the base terminal current of the transistor 107.
Conversely, when the oscillation output decreases, the base voltage increases corresponding to a decrease in the voltage drop across the resistance 120—this increases the gain of the transistor 107 and the oscillation output, maintaining the base current at a certain level determined by respective circuit constants. Because of its complicated configuration, the circuit of this example is used only in relatively expensive OCXOs, and is rarely used in common oscillators. On the other hand, there has been proposed such an oscillation circuit as depicted in FIG. 24 which is simple-structured but permits reduction in the current flow through the crystal vibrator. In this circuit denoted generally by 200, a parallel tuning circuit composed of a capacitance Cc and an inductance Lc is connected between the collector of an oscillation transistor TR1 and the power supply line Vcc AC-wise grounded via a capacitance Cv; a capacitance c3 is connected between the collector and emitter of the transistor; a parallel circuit of a capacitance Ce and a resistance Re is connected between the emitter of the transistor and the ground; and a crystal vibrator X is connected between the base of the transistor and the ground. An appropriate base bias voltage is applied by resistances Rb1 and Rb2. Incidentally, the oscillation output is derived via a secondary inductance of the above-mentioned inductance Lc. This circuit can be expressed equivalently as shown in FIGS. 25 and 26.
FIG. 25 shows an equivalent circuit of the crystal oscillator 200 of FIG. 24, in which z1 denotes a base-emitter interterminal impedance composed of a parallel circuit of a junction capacitance Cπ between the base and emitter of the transistor TR1 and an input resistance Rπ of the transistor TR1, z2 denotes an impedance composed of a parallel circuit of the resistance Re and the capacitance Ce between the emitter of the transistor TR1 and the ground, and gm denotes the mutual inductance of the transistor.
FIG. 26 shows an equivalent circuit of the crystal oscillator 200 in which the impedance z1 in FIG. 25 is replaced with a series circuit of a capacitance c1 component and a resistance r1, the impedance z2 in FIG. 26 is replaced with a capacitance c2 component and a resistance r2 component, and the parallel circuit of the capacitance Cc and the inductance Lc is converted to a capacitance component c4 (where c4=Cc−1/(ω2Lc)).
In FIG. 26 the input impedance ZIN of the transistor TR1 is ZIN=RIN+jXIN, and it can be expressed by the following equation by use of the circuit parameters shown in FIG. 26.                               Z          IN                =                ⁢                              R            IN                    +                      jX            IN                                                            R                      IN            ⁢                                                                =                ⁢                              r            1                    +                                                    g                m                                                                                  (                                                                  c                        3                                                                    c                        α                                                              )                                    2                                +                                                      (                                          ω                      ⁢                                                                                          ⁢                                              c                        3                                            ⁢                                              r                        2                                                              )                                    2                                                      ⁢                          {                                                                                          c                      3                                                              c                      α                                                        ⁢                                      (                                                                                            r                          1                                                ⁢                                                  r                          2                                                                    -                                              1                                                                              ω                            2                                                    ⁢                                                      c                            1                                                    ⁢                                                      c                            2                                                                                                                )                                                  -                                                      c                    3                                    ⁢                                                            r                      2                                        ⁡                                          (                                                                                                    r                            1                                                                                c                            2                                                                          +                                                                              r                            2                                                                                c                            1                                                                                              )                                                                                  }                                                                        X          IN                =                ⁢                              -                          1                              ω                ⁢                                                                  ⁢                                  c                  1                                                              -                                                    g                m                                                                                  (                                                                  c                        3                                                                    c                        α                                                              )                                    2                                +                                                      (                                          ω                      ⁢                                                                                          ⁢                                              c                        3                                            ⁢                                              r                        2                                                              )                                    2                                                      ⁢                          {                                                ω                  ⁢                                                                          ⁢                                      c                    3                                    ⁢                                                            r                      2                                        ⁡                                          (                                                                                                    r                            1                                                    ⁢                                                      r                            2                                                                          -                                                  1                                                                                    ω                              2                                                        ⁢                                                          c                              1                                                        ⁢                                                          c                              2                                                                                                                          )                                                                      +                                                                            c                      3                                                              ω                      ⁢                                                                                          ⁢                                              c                        a                                                                              ⁢                                      (                                                                                            r                          1                                                                          c                          2                                                                    +                                                                        r                          2                                                                          c                          1                                                                                      )                                                              }                                                                                    1                          c              α                                =                    ⁢                                    1                              c                2                                      +                          1                              c                3                                      +                          1                              c                4                                                    ,                              c            4                    =                    ⁢                                                    C                c                            -                                                1                                                            ω                      2                                        ⁢                                          L                      c                                                                      ⁢                                                                  ⁢                                  r                  1                                                      =                        ⁢                                          R                π                                            1                +                                                      (                                          ω                      ⁢                                                                                          ⁢                                              C                        π                                            ⁢                                              R                        π                                                              )                                    2                                                                    ,                                                      c            1                    =                    ⁢                      1                                          ω                2                            ⁢                              C                π                            ⁢                              R                π                            ⁢                              r                1                                                    ,                              r            2                    =                    ⁢                                    R              e                                      1              +                                                (                                      ω                    ⁢                                                                                  ⁢                                          C                      e                                        ⁢                                          R                      e                                                        )                                2                                                    ,                              c            2                    =                    ⁢                      1                                          ω                2                            ⁢                              C                e                            ⁢                              R                e                            ⁢                              r                1                                                        
FIG. 27 shows frequency characteristics of the negative resistance and reactance of an oscillation amplifying circuit portion of the crystal oscillator 200 of FIG. 24 which were obtained by simulations based on the above equation.
As shown in FIG. 27, since the frequency band in which to provide a sufficient value of negative resistance is as narrow as 9 MHz to 10 MHz, and since in this frequency band the reactance component abruptly changes from a capacitive to inductive one, it is difficult for the crystal oscillator 200 to keep on with stable oscillation; furthermore, the inductance connected to the collector of the oscillation transistor causes instability in its oscillation, readily leading to the occurrence of abnormal oscillation.
Accordingly, the oscillation circuit of such a configuration is altogether impracticable except for experimental use alone. As a mater of fact, it is only the AGC circuit in OCXO of FIG. 23 that can be put to practical use.
In the case of such a piezoelectric oscillation circuit 100 as described above, however, since the AGC circuit is complex in configuration and uses many parts, the crystal oscillator inevitably becomes bulky and there are limits on the reduction of the consumption of current and on the reduction of the current flow through the crystal vibrator. That is, though complex in circuit configuration, the above-described conventional AGC circuit type crystal vibrator current reducing means is applicable to the expensive oven type crystal oscillator (OCXO), but it cannot ever be applied to oscillators for ordinary portable telephones and radio equipment because the use of such means entails increased geometry of the device, a steep rise in manufacturing costs, increased consumption of current, and so forth.
The present invention is directed to the solution of the above-mentioned problems of the piezoelectric oscillation circuit, and has for its object to provide a piezoelectric oscillator which achieves improved secular change characteristics and effective suppression of undesired resonance through reduction of the excitation level of the crystal oscillator by use of a simple circuit configuration.